Help - Chimney Question

[Greg in Alaska]

Gentlemen,
Thank you for the advice you gave me last week with my high efficiency fireplace questions. I have another one for you on chimneys installation that the experts on this site may be able to help with. I searched the previous posts, and found lots of chimney posts, but nothing that answered my question. I bought my HE fireplace on saturday. I haven't picked it and the stovepipe up yet, but I went thru the piping detail with the salesperson to get the list of what we need. I am a bit concerned about how the stovepipe is to be installed and would like your opinion.
The stove will be installed inside a log house. The stovepipe (excel insulated 8" pipe) will run up thru a cathederal ceiling. Above the ceiling on the roof there is a chimney chase that goes up 10'. The salesman says I need a 2" clearance to combustable materials going thru the ceiling/roof line, and into the chase. His suggestion is to cut my hole thru the ceiling, and box it in to prevent the existing insulation from slumping against the chimney pipe, and to install a firestop on the inside of the ceiling against the drywall. You are probably famaliar with the firestop, it is basically a flat sheet of metal with a hole in it that fits around the chimney pipe. The problem, or I should say question, is that this firestop doesn't fit snug up against the chimney, so there is a fair bit of slop. It looks like a way to loose alot of heat from the house. It doesn't make much sense to me to install a high efficiency stove, then hack a great honking hole in the roof that is going to let a bunch of heat out of the house. I asked the sales man about filling the cavity with rock wool or something as the pipe goes thru the roof to replace the insulation, and to stop the air loss, but he said in no way should I put anything around the chimney in this space. His comment was that the exterior flashing would stop any air flow up thru the roof, and that insulation was not needed in this space. I am a bit skeptical about the air tightness of exterior rain flashing. What do you think. How do I run the chimney thru the roof without loosing all the heat in the house? Thanks in advance for the feedback.
Cheers,
Greg

 

[jtp10181]

Wood burning firestops cannot be caulked or sealed. So... they are having you just continue with the same pipe all the way up and out through the roof? How are you going to make it look nice as it goes through the ceiling? Just pipe going through a firestop will be quite ugly. We use catherdral ceiling support boxes to give it a better look. Also when you go through the ceiling into the insulation you should use an attic insulation shield to prevent the insulation from falling onto the pipe.

 

[BrotherBart]

He may be right. This is from the Excel installation manual:

"6. Do not fill the air space around the chimney with insulation or any
other material. Do not fill the factory built supports or radiation shields
with insulation. Insulation placed in this area could cause adjacent
combustibles to overheat."

The whole manual is here:

(broken link removed to http://www.icc-rsf.com/Manuel_catalogue/XLUSA-II.pdf)

 

[jtp10181]

I could get some pictures of houses that have burned down because someone didn't want to use an attic shield to keep the insulation off the pipe. He is definetly right, you need to keep that an airspace clearance on any stove, I can tell you this without looking in any manual. The airspace is desgiend to allow the pipe to keep cool. If you pack insulation around it (even if its non-combustible) the other combustibles around it that otherwise would have had proper clearance are now going to get hottter than usual. After a few years of this they will become so dry thier ignition temps will drop and eventually burst into flames.

 

[elkimmeg]

good advice here: That pipe requires 2" air space, Can you post a picture here showing the extent of your concerns. It might be as simple as cutting another piece of sheet metal tighter to the chimney pipe.

 

[Gooserider]

I could be wrong, but I believe there are "support boxes" that are intended to use when going through the ceiling. They are intended in part to support the weight of the chimney pipe, and as I understand it, also will act to at least somewhat contain your heat.

Gooserider